Also the "average" person won't maintain 2 social profiles and will gravitate to where the users already are.
The MySpace to Facebook transition shows that exactly this happened. I was in high school at the time, and I definitely remember people mentioning having both a MySpace and Facebook account, and wanting to get people to switch over to Facebook. I don't mean "tech people", I mean normal folk.
As for compelling features, I think Circles is a killer. When your relatives are on Facebook, a good portion of your personality has to leave Facebook. You have to think about what you're going to say, and generally stick to the blandest possible expression of who you are. If Google+ enables people to say what they want to who they want, it will be a hit.